EP0373976A1 - Monokristall-Szintillationsdetektor mit Lutetiumorthosilikat - Google Patents

Monokristall-Szintillationsdetektor mit Lutetiumorthosilikat Download PDF

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Publication number
EP0373976A1
EP0373976A1 EP89402755A EP89402755A EP0373976A1 EP 0373976 A1 EP0373976 A1 EP 0373976A1 EP 89402755 A EP89402755 A EP 89402755A EP 89402755 A EP89402755 A EP 89402755A EP 0373976 A1 EP0373976 A1 EP 0373976A1
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EP
European Patent Office
Prior art keywords
scintillator
approximately
detector
lso
range
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Application number
EP89402755A
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English (en)
French (fr)
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EP0373976B1 (de
Inventor
Charles L. Melcher
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Services Petroliers Schlumberger SA
Schlumberger NV
Schlumberger Ltd USA
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Societe de Prospection Electrique Schlumberger SA
Schlumberger NV
Schlumberger Ltd USA
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Application filed by Societe de Prospection Electrique Schlumberger SA, Schlumberger NV, Schlumberger Ltd USA filed Critical Societe de Prospection Electrique Schlumberger SA
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
    • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
    • G01T1/16—Measuring radiation intensity
    • G01T1/20—Measuring radiation intensity with scintillation detectors
    • G01T1/202—Measuring radiation intensity with scintillation detectors the detector being a crystal
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
    • G01V5/00—Prospecting or detecting by the use of ionising radiation, e.g. of natural or induced radioactivity
    • G01V5/04—Prospecting or detecting by the use of ionising radiation, e.g. of natural or induced radioactivity specially adapted for well-logging
    • G01V5/08—Prospecting or detecting by the use of ionising radiation, e.g. of natural or induced radioactivity specially adapted for well-logging using primary nuclear radiation sources or X-rays
    • G01V5/10—Prospecting or detecting by the use of ionising radiation, e.g. of natural or induced radioactivity specially adapted for well-logging using primary nuclear radiation sources or X-rays using neutron sources
    • G01V5/101—Prospecting or detecting by the use of ionising radiation, e.g. of natural or induced radioactivity specially adapted for well-logging using primary nuclear radiation sources or X-rays using neutron sources and detecting the secondary Y-rays produced in the surrounding layers of the bore hole

Definitions

  • the present invention relates to a single crystal scintillation detector for gamma rays and like radiation and, more particularly, to a single crystal scintilla­tion detector composed of lutetium orthosilicate.
  • a well-known form of detector for gamma rays and like radiation (such as x-rays, cosmic rays, and ener­getic particles of approximately 1 KeV and above) em­ploys a transparent single crystal, known as a scin­tillator, which responds to impinging radiation to emit light pulses.
  • the light pulses are optically coupled to the input of a photomultiplier tube, which generates a voltage signal related to the number and amplitude of the light pulses received.
  • Scintillators of this class have found wide application in various fields, such as nuclear medicine, physics, chemistry, mineral and petro­leum exploration, etc.
  • NaI detectors have found general use, for example, in log­ging tools for oil well logging operations, where either naturally occurring or induced gamma radiation is de­tected to aid in the location of petroleum deposits.
  • ganic scintillators such as naphthalene, anthracene, stilbene and similar materials, have also been employed, particularly where very high count rates are important, although they generally are not as useful as inorganic scintillators for the detection of gamma rays.
  • a gamma ray detector employing a scintillator formed of a single crystal of cerium-­activated gadolinium orthosilicate (GSO) has been pro­posed.
  • GSO scintillator has the advantages as a gamma ray detector of high effective atomic number, high density, fast scintillation decay, relatively low index of refraction, but has the disadvantages of low light output, a strong tendency to cleave which makes cutting and polishing difficult, and, more significantly, very high thermal neutron capture cross section (49,000 barns).
  • GSO scintillators would have very limited utility, if any, in those applications, such as many nuclear well logging tools for instance, where the gamma radiation to be detected is induced by neutron irradiation. This is because gadolinium, upon the capture of thermal neu­trons, emits gamma radiation which would interfere with the detection of the external gamma rays of interest.
  • Such phosphors are typically used as coatings on cathode ray tube screens, fluorescent light bulbs, and the like, where they con­vert impinging electrons or ultraviolet radiation into visible light pulses. They are, however, not suitable as gamma ray or like radiation detectors since energetic photons or particles have high probability of passing through the thin coating with no interaction. If the coating is made sufficiently thick to stop gamma rays, the resulting opacity of the phosphor layer would trap most of the scintillation signal. Consequently, only transparent single crystals are useful as gamma ray detectors.
  • an improved scintillator for use as a gamma ray (or like radiation) detector which comprises a single crystal of cerium-activated lutetium oxyorthosilicate having the general formulation Ce 2x Lu 2(1-x) SiO5.
  • the value of x (as measured in the initial melt from which the crystal is pulled) may be varied within the approximate range of from 0.001 to 0.1, with the preferred range of x being from approximately 0.005 to 0.015.
  • the scintillator crystal When assembled in a complete detector, the scintillator crystal is optically coupled, either directly or through a suitable light path, to the photosensitive surface of a photodetector for generation of an electrical signal in response to the emission of a light pulse by the scintillator.
  • the LSO scintillator of the invention possesses certain important characteristics, most notably high light output, very short decay time and high detection efficiency, that make it superior to prior scintillators as a gamma ray or like radiation detector, particularly in the borehole logging environment.
  • a single crystal LSO scintillator 10 is shown encased within the housing 12 of a gamma ray de­tector.
  • One face 14 of the scintillator is placed in optical contact with the photosensitive surface of a photomultiplier tube 16.
  • the light pulses could be coupled to the photomultiplier via light guides or fibers, lenses, mirrors, or the like.
  • the photomultiplier can be replaced by any suitable photodetector such as a photodiode, microchannel plate, etc.
  • the other faces 18 of the scintillator are preferably surrounded or covered with a reflective material, e.g.
  • Teflon tape magnesium oxide powder, aluminum foil, or titanium dioxide paint.
  • Light pulses emitted by the LSO crystal upon the incidence of radiation are intercepted, either directly or upon reflection from the surfaces 18, by the photomultiplier, which generates electrical pulses or signals in response to the light pulses. These electrical output pulses are typically first amplified and then subsequently processed as desired, e.g. in a pulse height amplifier, to obtain the parameters of interest regarding the detected radiation.
  • the photomultiplier is also connected to a high voltage power supply, as indicated in Fig. 1.
  • Fig. 1 Other than the LSO scintillator, all of the components and materials referred to in connection with Fig. 1 are conventional, and thus need not be described in detail.
  • the phosphor synthesis procedure consisted of the following steps:
  • Table 2 shows the intensity of the fluorescence emission measured at room temperature. TABLE 2 GSO:CeO2 GSO:Ce2O3 LSO:CeO2 LSO:Ce2O3 Argon + H2 44 17 71 27 air 1.8 1.6 11 28 Argon 12 8.5 100 86
  • Table 3 summarizes the scintillation properties of GSO and LSO phosphors, synthesized in the foregoing man­ner: TABLE 3 GSO:CeO2 LSO:CeO2 Light output 1 1.5-10 Decay time 60 ns 50 ns Emission peak 430 nm 415 nm Temperature response -0.4%/°C -1.3%/°C
  • the scintillation decay time of LSO of about 50 ns compares favorably with GSO's 60 ns.
  • the effective atomic number of LSO is 66 compared to 59 for GSO and the density of LSO is 7.4 gm/cc compared to 6.7 gm/cc for GSO, both of which contribute to a higher radiation detection efficiency for LSO.
  • the index of refraction of LSO is 1.82 compared to 1.91 for GSO, which results in less trapping of scintillation light.
  • LSO is very much less sensitive to neutrons, since the thermal cross section for Lu is 77 barns compared to 49,000 barns for Gd.
  • the temperature response of LSO is somewhat worse than GSO. At 150°C, GSO's light output decreases to about 60% of its room temperature value, while LSO's light output decreases to about 20% of its room temperature value at 150°C.
  • the emission spectrum of LSO was found to shift somewhat to shorter wavelengths than the GSO emission as temperature increased from room temperature up to 175°C (the highest temperature measured). Again this would be advantageous in the single crystal form in terms of matching photomultiplier response.
  • Lutetium has a radioactive isotope (176Lu) that produces a background noise level in the crystal. This could be eliminated by growing the crystal from pure 175Lu, or it could be handled by conventional background subtraction techniques.
  • the excitation spectrum of LSO exhibits three bands (262 nm, 298 nm, and 355 nm) and is similar to the GSO excitation spectrum except that the bands are shifted to somewhat longer wavelengths.
  • Table 4 summarizes the scintillation properties of the LSO single crystals grown, as selected and cut to minimize imperfections.
  • the composition of the melts was Ce 2x Lu 2(1-x) SiO5, where x is the decimal value of the percentage set out in Table 4 under the heading "Ce nom”.
  • Cerium concentration in the crystals was on the order of 20%-30% of that in the melt. TABLE 4 size color defects Ce nom.
  • Crystals 1, 2 and 4 were cut to the sizes listed from larger single crystals (5mm x 6mm x 28mm, 7mm x 9mm x 27mm and 8mm x 8mm x 33mm, respectively), but crystal 3 was the original size. All were clear of color and of high transparency, but crystals 2 and 4 had some defects.
  • the light output was measured by coupling each crystal with optical coupling grease directly to a Hamamatsu R878 photomultiplier, with all surfaces except that coupled to the photomultiplier covered with Teflon tape (crystal 3) or titanium dioxide paint (crystals 1, 2 and 4).
  • the scale employed for the light output mea­surements is expressed in arbitrary units.
  • the light output of a standard NaI (Tl) scintillator would be on the order of 200, and that for a standard GSO scintillator would be on the order of 40.
  • the energy resolution was determined by using a standard cesium 137 gamma ray source. The energy resolution is expressed as the full width at half-maximum of the 662 KeV gamma ray peak.
  • the scin­tillation decay time was exponential and had an average value of about 42 ns among the four crystals, as mea­sured by the time-correlated, single photon technique.
  • the emission spectrum under gamma excitation was found to be different from the emission spectrum under ultraviolet excitation.
  • the gamma emis­sion spectrum peaked at approximately 426 nm -430 nm and was similar to the GSO emission spectrum.
  • both the gamma and ultraviolet-excited emissions exhibited a thermoluminescent effect with a half-life of about 10 minutes.
  • the temperature response of the LSO crystals was not as good as GSO.
  • the gamma-excited emission fell off at approximately 1.3% per degree C.
  • the peak output is 20% of that at room temperature, which is similar to the temperature response of BGO. Accordingly, in those applications where high tempera­tures are anticipated, such as in certain oil well log­ ging tools, the LSO scintillator may need to be isolated from the environment by a Dewar flask or other insulator.
  • LSO single crystal scintillators can be produced with cerium concentrations (in the melt from which the crystal is pulled) within the approximate range of from 0.1% to 10%, i.e., 0.001 ⁇ x ⁇ 0.1.
  • the preferred melt cerium concentration is within the range of from approximately 0.5% to 1.5%, i.e. 0.005 ⁇ x ⁇ 0.015.
  • Table 5 compares the principal physical and scintillation properties of the LSO single crystals with those for NaI(Tl), BGO, and GSO.
  • NaI(Tl) crys ­tal arbitrarily assigned a reference light output value of 100, it may be seen that the LSO crystal at 75 is markedly superior to the BGO and GSO crystals and only 25% below the NaI(Tl) value.
  • the energy resolution of the LSO scintillator compares quite favorably with BGO and GSO and, again, was only slightly worse than NaI(Tl).
  • the signal-to-noise performance of the LSO scintillator therefore, is much improved relative to the BGO and GSO detectors.
  • LSO possesses other properties that are superior to NaI(Tl).
  • the average decay time of 41 ns is shorter than any of the other three crystals and is some 5 to 6 times shorter than NaI(Tl).
  • the LSO scintillator, there­fore, is particularly useful in high counting rate de­tectors.
  • LSO also has a very high gamma ray detection efficiency by virtue of its high effective atomic number and density. It is superior in this respect to both NaI(Tl) and GSO and is comparable to BGO. High detec­tion efficiency further contributes to LSO's suitability for high counting rate applications.
  • LSO low index of refraction
  • LSO is also non-hygroscopic, a par­ticular advantage for oil well logging applications or other wet environments. Its mechanical ruggedness is superior to both NaI(Tl) and GSO, a feature which is also desirable for well logging and other uses where rough handling is encountered.
  • the gamma emission peak is at ap­proximately 428 nm, which is substantially the same as GSO and only slightly above NaI(Tl).
  • the neutron cross section is especially favorable in comparison to GSO, 84 barns vs. 49,000 barns. Hence the occurrence of interfering gamma rays due to neutron cap­ture within the crystal is greatly reduced relative to GSO.
  • the radiation length of LSO is as good as that of BGO and considerably shorter than either GSO or NaI, with consequent advantages in the crystal size re­quired.
  • the LSO single crystal scintillator is comparable to or exceeds other known scintillators in nearly all properties im­portant for use as a gamma ray detector, i.e., light output, energy resolution, efficiency of detection of high energy photons, scintillation decay time, hygro­scopicity, susceptibility of crystal to mechanical dam­age, refractive index, emission spectrum match to photo­multiplier tube response, transparency of the crystal to its own scintillation emission, and absence of induced gamma radiation within the crystal.
  • the only area in which LSO compares unfavorably is in the temperature sensitivity of the gamma-excited emission. In controlled environments, e.g.
  • the LSO scintillator detector of the present invention is particularly effective as a radiation detector in a borehole logging environment, such as for petroleum exploration.
  • the detector forms part of a logging system which may be of the type disclosed in the aforementioned copending application Serial No. 149,953 and illustrated in Figure 2 herein.
  • Figure 2 shows a logging sonde 11 for sensing gamma radiation resulting from bombardment of a formation with high energy neutrons and detecting the energy of the radiation for subsequent spectral analysis.
  • the sonde 11 is suspended in a borehole 13 on an armored multi­conductor cable 15.
  • the borehole 13 traverses a formation 17 and is filled with fluid 19, and may be open as shown or cased.
  • the sonde 11 as described below may be constructed in accordance with U.S. Patent No. 4,317,993 to Hertzog, Jr. et al, assigned to the assignee of the present application.
  • the sonde 11 is moved in the borehole 13 by playing the cable 15 out and reeling it back in over a sheave wheel 20 and a depth gauge 22 by means of a winch forming part of a surface equipment 24.
  • a winch forming part of a surface equipment 24.
  • the logging measurements are actually made while the sonde 11 is being raised back up the borehole 13, although in certain circumstances they may be made on the way down instead or as well.
  • the sonde 11 includes a pulsed neutron source 26 for producing primary radiation to bombard the formation 17 with fast neutrons as the sonde 11 travels up the borehole 13, and a radiation detector 28 for detecting secondary (gamma) radiation induced thereby in the borehole 13 and the formation 17.
  • the neutron source 26 is preferably of the pulsed accelerator type described in U.S. Patents No. 3,461,291 to Goodman and No. 3,546,512 to Frentrop, both commonly owned with this application. This type of source is particularly suited to the generation of discrete bursts of high energy or fast neutrons, e.g. at 14 MeV, with a controlled duration and repetition rate.
  • the detector 28 is of a type appropriate to the detection of gamma radiation and the production of an electrical signal corresponding to each detected gamma ray and having an amplitude representative of the energy of the gamma ray. To this end the detector 28 is as shown in Figure 1, including a cerium-activated LSO scintillation crystal 10 optically coupled to a photomultiplier tube (PMT) 16. Suitable tubes are manufactured by EMR Photoelectric, Princeton, New Jersey.
  • a neutron shield 34 may be located between the source 26 and the detector 28 to limit direct bombardment of the detector 28 by neutrons from the source 26, thereby avoiding saturation of the detector 28 by such direct irradiation.
  • the sonde 11 may be surrounded by a sleeve 36 impregnated with boron carbide and located in the general vicinity of the source 26 and the detector 28. This sleeve displaces borehole fluid in the region of the detector 28, and absorbs neutrons scattered by the formation towards the detector 28, without significantly attenuating gamma radiation emanating from the formation.
  • the net effect is to reduce the possiblity of neutron interactions with the borehole contents and the material of the sonde 11 in proximity to the detector 28 and which would otherwise produce detectable gamma rays constituting an undesirable perturbation of the required gamma ray measurement.
  • the sonde 11 includes power conditioning circuitry (not shown) for feeding power at appropriate voltage and current levels to the source 26, the detector 28 and other downhole circuits. These circuits include an amplifier 38 and associated circuitry which receives the output pulses from the PMT 16. The amplifed pulses are then applied to a pulse height analyzer (PHA) 40 including an analog-to-digital converter which may be of any conventional type such as the single ramp (Wilkinson rundown) type. Other suitable analog to digital converters may be used for the gamma ray energy range to be analyzed. Linear gating circuits may also be employed for control of the time portion of the detector signal frame to be analyzed. Improved performance can be obtained by the use of additional conventional techniques such as pulse pile-up rejection.
  • PHA pulse height analyzer
  • the pulse height analyzer 40 assigns each detector pulse to one of a number (typically in the range 256 to 8000) of predetermined channels according to its amplitude (i.e. the gamma ray energy), and produces a signal in suitable digital form representing the channel or amplitude of each analyzed pulse.
  • the pulse height analyzer 40 includes memory in which the occurrences of each channel number in the digital signal are accumulated to provide an energy spectrum. The accumulated totals are then transferred via a buffer memory 42 (which can be omitted in certain circumstances) to telemetry and cable interface circuits 44 for transmission over the cable 15 to the surface equipment 24.
  • the cable signals are received by cable interface and signal processing circutis 46.
  • the circuits 44 and 46 may be of any suitable known construction for encoding and decoding, multiplexing and demultiplexing, amplifying and otherwise processing the signals for transmission to and reception by the surface equipment 24. Appropriate circuits are described, for example, in U.S. Patent No. 4,012,712 to Nelligan.
  • the operation of the sonde 11 is controlled by signals sent downhole from a master programmer 48, located in te surface equipment 24. These signals are received by a tool programmer 50 which transmits control signals to the neutron source 26 and the pulse height analyzer 40.
  • the surface equipment 24 includes various electronic circuits used to process the data received from the downhole equipment, analyze the energy spectrum of the detected gamma radiation, extract therefrom information about the formation 17 and any hydrocarbons that it may contain, and produce a tangible record or log of some or all of this data and information, for example on film, paper or tape.
  • These circuits may comprise special purpose hardware or alternatively a general purpose computer appropriately programmed to perform the same tasks as such hardware. Details of such analysis form no part of this invention and will not be described here, but may be found for example in U.S. Patent No. 3,521,064.

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EP89402755A 1988-10-06 1989-10-05 Monokristall-Szintillationsdetektor mit Lutetiumorthosilikat Expired - Lifetime EP0373976B1 (de)

Applications Claiming Priority (4)

Application Number Priority Date Filing Date Title
US25435388A 1988-10-06 1988-10-06
US07/389,502 US4958080A (en) 1988-10-06 1989-08-04 Lutetium orthosilicate single crystal scintillator detector
US389502 1989-08-04
US254353 2002-09-25

Publications (2)

Publication Number Publication Date
EP0373976A1 true EP0373976A1 (de) 1990-06-20
EP0373976B1 EP0373976B1 (de) 1993-01-13

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EP89402755A Expired - Lifetime EP0373976B1 (de) 1988-10-06 1989-10-05 Monokristall-Szintillationsdetektor mit Lutetiumorthosilikat

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US (1) US4958080A (de)
EP (1) EP0373976B1 (de)
JP (1) JP2852944B2 (de)
DE (1) DE68904408T2 (de)
NO (1) NO304287B1 (de)

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DE68904408D1 (de) 1993-02-25
DE68904408T2 (de) 1993-07-29
EP0373976B1 (de) 1993-01-13
NO304287B1 (no) 1998-11-23
JPH02225587A (ja) 1990-09-07
NO893973D0 (no) 1989-10-05
US4958080A (en) 1990-09-18
JP2852944B2 (ja) 1999-02-03
NO893973L (no) 1990-04-09

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